1. Field of the Invention
The present invention relates to a system for determining the passability of a vehicle, which involves perceiving a shape of a road based upon map data comprised of an aggregation of a plurality of nodes and determining whether the vehicle can safely pass through the nodes.
2. Description of the Related Art
A system for determining the passability of a vehicle is known from Japanese Patent Application Laid-open No. 5-141979, wherein a radius of curvature of a curved section of a road is presumed by calculating a radius of an arc passing through three nodes, a passable speed of the vehicle is calculated based on the radius of curvature of the curved road section, a passage-predicting speed of the vehicle is calculated based on a current vehicle speed, and the passable vehicle speed is compared to the passage-predicting speed of the vehicle. If the passage-predicting speed is equal to or lower than the passable speed, it is determined that the vehicle can safely pass through the curve.
In the known system identified above, if three nodes N.sub.1, N.sub.2 and N.sub.3 exist on the same curve, as shown in FIG. 12A, a radius R of the curve can be correctly presumed. However, the above conventional system suffers from the following problem: if middle node N.sub.2 exists on a curve and leading node N.sub.1 and trailing node N.sub.3 are out of the curve, as shown in FIG. 12B, a radius R' of curvature of the curve presumed from the three nodes N.sub.1, N.sub.2 and N.sub.3 is larger than an actual radius R of curvature of the curve, thereby incorrectly calculating the shape of the curve.
In cases of simple curves having a constant radius of curvature (curves whose direction of curvature does not change from the curve inlet to curve outlet), it is more difficult for a vehicle to pass through the curve b having a longer curved-section than through the curve a having the same radius of curvature but a shorter curved-section, as shown in FIG. 13A. In cases of curves having a constant radius of curvature, it is more difficult for a vehicle to pass through an S-shaped curve (curve c, whose curving direction changes from the right to the left or from the left to the right) than through the simple curve having the same radius of curvature (curve b, as shown in FIG. 13B). Namely, the passable speeds for a vehicle passing through the curves are such the speed through curve a is greater than the vehicle speed through curve b which is greater than vehicle speed through curve c. In the above-identified, known system, however, if the radii of curvature of curves a, b and c are equal to each other, the passable speeds through the curves are computed to be equal to each other. For this reason, it is difficult to accurately determine whether the vehicle can pass through a curve based upon the shape and type of the curve.